Particulate, absorbent, polymeric compositions are capable of absorbing large quantities of liquids such as water and body exudates (e.g., urine) and are further capable of retaining such absorbed liquids under moderate pressures. The absorption characteristics of such polymeric compositions make them especially useful for incorporation into absorbent articles such as diapers. See, for example, U.S. Pat. No. 3,669,103 (Harper et al), issued Jun. 13, 1972, and U.S. Pat. No. 3,670,731 (Harmon), issued Jun. 20, 1972, that disclose the use of particulate, absorbent, polymeric compositions (often referred to as "hydrogels", "superabsorbents", or "hydrocolloid materials") in absorbent articles.
Conventional particulate, absorbent, polymeric compositions, however, have the limitation that the particles are not immobilised and are free to migrate during processing and/or use. Migration of the particles can lead to material handling losses during manufacturing as well as non-homogeneous incorporation of the particles into structures in which the particles are being used. Especially when the absorbent polymer particles are incorporated in a fibrous matrix at high concentrations, the particles may sift out of the matrix of may become inhomogeneously distributed in an uncontrolled manner, as for instance described in European application number 94111955.4 (Bogdanski et. al.) Another significant problem occurs when these particulate materials migrate during or after swelling in use. Such mobility leads to high resistance to liquid flow through the material due to the lack of stable interparticle capillary or liquid transport channels. This phenomenon is one form of what is commonly referred to as "gel blocking."
One attempt to overcome the performance limitations associated with absorbent particle mobility during use in absorbent articles is incorporation of the particulate, absorbent, polymeric compositions into tissue laminates, i.e. layered absorbent structures. By encapsulating the particles between tissue layers, and affixation of the particles by water bonding or glue bonding, the overall particle mobility within an absorbent structure is diminished. However, upon liquid contact, the particles within the laminate are often free to move relative to each other resulting in the breakdown of any pre-existent interparticle capillary channels.
Another attempted solution is to immobilise the particulate, absorbent, polymeric compositions by the addition of large quantities of liquid polyhydroxy compounds that act as an adhesive to hold the particles together or attach the particles to a substrate. See, for example, U.S. Pat. No. 4,410,571 (Korpman), issued Oct. 18, 1983. While this approach does limit migration before and, to some extent, during swelling, the particles eventually become detached from each other or from the substrate in the presence of excess liquid, resulting again in the breakdown of any pre-existing capillary channels between the particles.
Another attempted solution to overcome the problem of absorbent particle mobility is to produce a hydrogel-forming film by extrusion of a solution of a linear absorbent polymer and subsequently crosslinking it. See, for example, U.S. Pat. No. 4,861,539 (Allen at al), issued Aug. 29, 1989 (crosslinked with a polyhydroxy compound such as a glycol or glycerol); and U.S. Pat. No. 4,076,673 (Burkholder), issued Feb. 28, 1978 (crosslinked with polyamine-polyamide epichlorohydrin adducts such as Kymene.RTM.). While these hydrogel-forming films may absorb significant quantities of liquids, they have limited liquid transport properties because they are essentially non-porous, i.e. lack internal capillary channels. Indeed, due to the lack of internal capillary channels, these hydrogel-forming films are especially prone to gel blocking.
A more recent solution proposed to overcome the problem of absorbent particle mobility is to form these particles into aggregate macrostructures, typically as sheets of bonded absorbent particles. See U.S. Pat. No. 5,102,597 (Roe et al), issued Apr. 7, 1992. These aggregate macrostructures are prepared by initially mixing the absorbent particles with a solution of a nonionic crosslinking agent, water and a hydrophilic organic solvent such as isopropanol. These nonionic crosslinking agents include polyhydric alcohols (e.g., glycerol), polyaziridine compounds (e.g., 2,2-bishydroxymethyl butanoltris[3-(1-aziridine) propionate]), haloepoxy compounds (e.g., epicholorhydrin), polyaldehyde compounds (e.g., glutaraldehyde), polyamine compounds (e.g., ethylene amine), and polyisocyanate compounds (e.g., 2,4-toluene diisocyanate), preferably glycerol. See Column 11, lines 22-54, of U.S. Pat. No. 5,102,597.
It has been described in U.S. Pat. No. 5,102,597 that the hydrogel-forming polymeric precursor particles may be brought into mutual contact by first depositing the particles on a substrate and subsequently effecting inter-particle crosslinking of the contiguous particles.
According to U.S. Pat. No. 5,124,188, reinforcement structures such as fibers, webs or scrims may be embedded into the macrostructures of crosslinked particles, to provide structural integrity. Embedding the fibers or webs into the macrostructure of crosslinked particles is effected by mixing the fibers with the solution containing the inter-particle crosslinking agent or by mixing the fibers with the particles prior to inter-particle cross-linking. Kneading the fibers into the interparticle crosslinking agent/precursor particle mix is preferred.
In U.S. Pat. No. 5,180,622 (Berg), the formation of an interparticle crosslinked aggregate is described, wherein the aggregate is joined to a carrier, which may be comprised of cellulosic fibers or which may be formed by a web. Joining the aggregate and the carrier is generally described as being effected via physical or chemical bonding using adhesives or chemicals that react to adhere the macrostructure, or inter-particle crosslinked aggregate, to the carrier.
The known formation of composites of combined interparticle crosslinked aggregates and reinforcing webs is relatively complex, and results in structures which comprise a relatively large weight of hydrogel-forming polymeric particles relative to the weight of the substrate or carrier. Hence the known composites are less suitable for absorbent products in which less absorbency is required, such as for instance pantiliners.
Also, joining of the interparticle crosslinked aggregate to a web, will lead to some degree of loss of flexibility of the combined substrate and aggregate. Composites of good flexibility may be obtained, although this requires addition of for instance a plasticiser to the interparticle crosslinked aggregates. However, it has been found that known interparticle crosslinked aggregates may lose water when stored for a larger period of time in a dry environment. This may then cause the aggregates to become stiff or brittle.
Finally, the known interparticle crosslinked aggregates are relatively fluid stable as they isotropically expand upon being wetted. However, the inter-particle crosslinked aggregates may form a resistance to vertical transmission of liquid through the absorbent structure, which in multilayer absorbent products as described in European application no. 93305150.0 and no 93309614.1 may lead to reduced acquisition or fluid uptake rates.
In EP-B-0 273 141 it has been described to adhere hydrogel-forming polymeric particles to fibers to form absorbent flocks. The particles are attached to the fibers by adhesive attachment or by mixing fibers and monomers followed by polymerisation to form the absorbent polymer in which the fibers are embedded. A preferred way of attaching fibers and particles is by dissolving the polymeric particles in water, mixing the fibers into the solution, drying the solution to solidify the polymeric material, and grinding the polymeric material-fiber composite to form flocks of the desired size.
The absorbent flocks have as a disadvantage that adhesive attachment of polymeric particles to the fibers may negatively impact on the absorptive properties of the particles. Furthermore, an absorbent product formed of a multiplicity of flocks will not isotropically swell upon being wetted and hence have a tendency to exhibit gel blocking.
It is an object of the present invention to form an absorbent composite structure comprising hydrogel-forming polymeric particles connected to a substrate which do not become detached from the substrate upon being wetted.
It is a further object of the invention to provide an absorbent composite structure in which the hydrogel-forming polymeric particles have a reduced tendency to exhibit gel blocking.
It is again a further object of the invention to provide an absorbent structure which comprises a relatively low basis weight of hydrogel-forming polymeric particles.
It another object of the invention to provide an absorbent composite structure which is flexible and which has relatively little resistance to vertical transmission of liquids.